John Calvin Commentary Acts 26:10

"And this I also did in Jerusalem: and I both shut up many of the saints in prisons, having received authority from the chief priests, and when they were put to death I gave my vote against them." — Acts 26:10 (ASV)

Which thing I did. He proves by his very actions with what forceful zeal he was carried away to strive against Christ, until a greater force pulled him back and made him go in the completely contrary direction. Furthermore, his adversaries were witnesses of his vehemence, so it was most certain that he was suddenly changed. Undoubtedly, the priests would never have put him in any such office unless he had behaved courageously in exercising cruelty, and it was fitting that he should be very courageous to satisfy their fury.

It is also to be noted that Paul was not ashamed to confess how severely he had offended against God, so that it might turn to the glory of Christ. It was undoubtedly reproachful for him: to have been carried away with blind zeal, so that he forced those to blaspheme who desired to serve God; to have troubled the good and simple in various ways; to have pronounced sentence for the shedding of innocent blood; finally, to have lifted up his horns even to heaven, until he was thrown down.

But he does not spare his own reputation, but willingly declares his own shame, so that the mercy of God may appear more plainly through it.

Therefore, no sinister suspicion could rest in his speech, seeing that (without any regard for himself), he says that he utterly offended in those things by which he gained the praise of all the people. Therefore, he condemns his very zeal as madness, which others honored.

By this it appears how filthy the ambition of those men is who are ashamed to simply confess if they have offended through ignorance or error. For although they do not altogether excuse it, yet they try to lessen or gloss over these things, for which they ought humbly with sorrow and tears to ask for pardon.

But though Paul might have retained the fame of a courageous man, yet he confesses he was a madman. For the participle that Luke uses implies this much: that he compelled many to blaspheme.

By this we know that there was great corruption even in the very first fruits of believers, seeing that having first professed themselves to be disciples of Christ, and being afterwards discouraged by fear or beatings, they not only denied him but also spoke evil of his blessed name. Though the very denial itself contains a horrible blasphemy.